1. Field of the Invention
The present invention relates in general to a charging and discharging control circuit having a terminal for charging and discharging control, and a charging type power supply unit. In particular, the invention relates to a charging and discharging control circuit in which the charging and discharging control terminal is made to have a test function as well so that a single external terminal has both a charging and discharging control function and the test function, and to a charging type power supply unit.
The present invention relates to a secondary battery protection circuit having both a charging and discharging control function and a test function, and a mass production technique for reducing a test time for a charging type power supply unit incorporating therein the secondary battery protection circuit.
2. Description of the Related Art
The features of a lithium ion secondary battery which has greatly contributed to the popularization of mobile devices typified by a mobile telephone and a PHS are its small size, lightweight, and a large capacity. Those features have led to realization of long time driving and lightness of the mobile devices. However, since a secondary battery is repeatedly charged and discharged, there is a high probability that it will become an overcharge state or an overdischarge state. If the secondary battery becomes the overcharge state, a battery temperature rises so that an internal pressure of the secondary battery is increased and metal Li is precipitated due to generation of a gas owing to decomposition of an electrolyte. Thus, there is a risk of ignition or explosion of the battery. Conversely, if the secondary battery becomes the overdischarge state, the electrolyte is decomposed to deteriorate the characteristics of the battery. In order to prevent such situations from occurring, a protection circuit is incorporated in a battery pack.
It has become the basic technique for a protection circuit to provide a charging and discharging control switch circuit in a charging and discharging path between a secondary battery and a main body of a mobile device, and to detect, by means of a charging and discharging circuit, charging of the secondary battery up to a level equal to or higher than a predetermined voltage, discharging of the secondary battery down to a level equal to or lower than a predetermined voltage, discharging of the secondary battery in the form of an excessive current, to turn OFF the charging and discharging control switch in order to prevent the secondary battery from becoming any of an overcharge state, an overdischarge state, and an overcurrent state.
In order to cope with a case where a battery pack is taken off from a main body of a mobile device and a case where charge and discharge prohibition control is required for the main body of the mobile device, a charging and discharging control terminal is provided in many secondary battery protection circuits.
Since a lithium ion secondary battery has a large internal impedance, the apparent battery voltage is changed due to a charge current and a discharge current. While the charge current is caused to flow, the apparent battery voltage is high. On the other hand, while the discharge current is caused to flow, the apparent battery voltage is low. For the purpose of efficiently using the battery, there is a need to provide a delay time for each of detection of overcharge and detection of overdischarge. In addition, for the purpose of preventing false release due to noises, there is a need to provide a release delay time. It is disclosed in JP 2001-283932 A (pp. 1 to 6 and FIG. 1), for example, to provide the above-mentioned delay times by an internal delay circuit to shorten test times for overcharge and overdischarge. Since this internal delay circuit provides all the delay times, there is no need to provide an external capacitor for determining a delay time, and as a result, it is possible to reduce the number of external components or parts required for a protection circuit.
However, in a charging and discharging control circuit using a built-in delay circuit, a delay time cannot be easily changed from the outside. Thus, it takes a great deal of time to evaluate the characteristics of the charging and discharging control circuit due to the delay times. Since delay times for detection of an overcurrent and detection of an overdischarge are generally in the range of about several msec to about several hundreds msec, it does not exert a very large on the test time. However, since the delay time for detection of an overcharge is normally set to about several seconds, it takes a lot of time to carry out the test. Consequently, there is a need to provide a test mode adapted to shorten a delay time in the charging and discharging control circuit using the built-in delay circuit.
In JP 2001-283932 A (pp. 1 to 6, and FIG. 1), there are disclosed a charging and discharging control circuit which enters a test mode in which a delay time of an internal control circuit is shortened when a voltage equal to or higher than a prescribed voltage is applied to a charger connection terminal, and a charging type power supply unit. FIG. 3 shows an embodiment of the invention described in JP 2001-283932 A. When a secondary battery becomes an overcharge state, an output signal from an overcharge detection comparator 113 goes to a high level, and an internal control circuit 120 outputs a control signal to an internal delay circuit 121. After a lapse of a delay time t1 as prescribed with the output voltage as an input signal, the internal delay circuit 121 outputs a signal for controlling a switch circuit 102.
Further, when a voltage at an overcurrent detection terminal has increased up to a level equal to or higher than a prescribed voltage V1, an output signal from a voltage detection comparator 115 goes to a high level. When the output signal from the voltage detection comparator 115 goes to the high level, the internal control circuit 120 enters a state in which the internal control circuit 120 is ready to output a control signal for shortening a delay time in the internal delay circuit 121, and maintains this state. When the secondary battery becomes the overcharge state, the output signal from the overcharge detection comparator 113 goes to the high level and then the internal control circuit 120 outputs the control signal to the internal delay circuit 121. After a lapse of a delay time t2 as prescribed with the output voltage as an input signal, the internal delay circuit 121 outputs a signal for controlling the switch circuit 102. As a result, once the voltage at the overcurrent detection terminal becomes a voltage equal to or higher than the prescribed voltage, the delay time remains shortened. Thereafter, the overcharge detection voltage can be measured in a state in which the overcharge delay time remains shortened.
On the other hand, when the voltage at the overcurrent detection terminal has decreased down to a level equal to or lower than a prescribed voltage V2, an output signal of a voltage detection comparator 114 goes to a high level. When the output signal of the voltage detection comparator 114 becomes the high level, the internal control circuit 120 releases the state in which it outputs the control signal for shortening the delay time which is provided by the internal delay circuit 121, thereby returning the current delay time back to the normal delay time t1. As a result, once the voltage at the overcurrent detection terminal becomes a level equal to or lower than the prescribed voltage V2, the test mode is released to provide the normal state.
In the invention disclosed in JP 2001-283932 A, shortening the internal delay time by utilizing the overcurrent detection terminal is effective for reduction of cost. However, this system is more inconvenient than a system in which an independent external terminal used for testing (hereinafter referred to as “test terminal”) is provided in order to control a delay time used for testing (hereinafter referred to as the “test delay time”). In particular, this system is even more inconvenient when used for a test of a secondary battery pack carried out at a customer's place. Moreover, in a case where it becomes necessary to divide an overcurrent detection voltage into plural levels for control, with the above-mentioned technique, there is encountered a problem in that the circuit configuration becomes complicated, making it impossible to cope with such case.
However, if a test terminal is further independently provided in the secondary battery protection circuit having the charging and discharging control terminal therein, then the number of pins for control is increased accordingly to increase cost.
In addition, in a case where an overcharge detection voltage is to be accurately measured when carrying out initial measurement for trimming to set voltages for the detection of overcharge and overdischarge and the release thereof at a plant, whenever an input voltage is stepped, a latency time equal to or larger than several seconds is required. Thus, even if a detection voltage can be measured in 25 steps, when a latency time is assumed to be 5 seconds, a time required for the measurement of the overcharge detection voltage becomes 125 seconds. Even if the system has a test mode in which a delay time is shortened to 1/50 of that time, the measurement takes as long as 2.5 seconds for one chip. Thus, it takes too much time to mass produce control circuits and hence this becomes a serious problem in terms of testing cost.
That is to say, there is a need to further shorten a detection delay time for the initial measurement made at a plant for a delay circuit built-in secondary battery charging and discharging control circuit. Also, both a delay time in normal use and a test mode adapted to shorten the delay time are required for the secondary measurement and evaluation made at a customer's place. It is a challenge to realize such a control function with few external terminals.